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Reasons for a Lunisolar Calendar

It is fairly easy to justify keeping a calendar that keeps track of the tropical year. Even in most tropical regions, there is still a yearly cycle of weather: it may be a dry season and wet season, but its still linked to the tropical year.

And given the nature of humans and what can be seen in the sky -- most notably, that only the moon shows phases visible to the unaided human eye --, it was likely inevitable that the synodic lunar month would be a part of so many human calendar systems.

However, the weather changes from seasons affect so many human activities that making a solar calendar system can be easily justified, especially away from the tropics.

But, some people have thought that a lunisolar calendar would be better, even I have thought this at times. I was an enthusiastic amateur astronomer when I was younger, so a calendar that also tracked the phases of the moon seemed like a neat idea. But now, I find it harder to justify, because what justification is there for tracking the phases of the moon than there is not for tracking the phases of Venus.

I have come up with only two justifications. First the moon can at some times of the lunar month cast usable light (unlike any other non-solar body). (Incidentally, astronomers would benefit from this being tracked by the calendar because of a kind of opposite of this: knowing when the sky will be dark just by looking at the date.)

Second, the tides. I know that the tides are more complicated than having high tide when the moon is on the meridian; however, there are many places that repeat similar patterns each lunar month, so sailors (especially those going to a limited number of ports) might be better able to keep tide tables in their head.

Can anyone else give any other practical reason for having a lunisolar calendar?

There was a very practical use of the lunar cycle in the battle against the German U-boats in the Atlantic Ocean during World War II. Captain (later Rear Admiral) Daniel V. Gallery pioneered night flying from his escort carrier USS Guadalcanal in the spring of 1944. This of course was dangerous, especially on the tiny flight decks of these "baby flattops". He started training the pilots under a full Moon, so they could see the ship clearly. Over the next two weeks as the Moon waned, the pilots gained the skill and confidence to land with minimal lighting on the ship. His fellow escort carrier captains followed the trail he blazed, and the subs could no longer come up for air safely at any time.

Last edited by Hornblower; 2020-Apr-08 at 03:10 AM.
Reason: Add name of ship

Iím not sure it requires adopting a lunisolar calendar rather than simply knowing the phases of the moon.

The "problem" with that argument is that one could equally argue that there is no necessity to adopt a solar calendar either; one could simply require using the right ascension or ecliptic longitude of the sun. In practice, perhaps an analog to the "age of the moon" (time since last new moon) would be used -- keeping track of the number of days since the last vernal equinox. We could eliminate the solar calendar and go to a straight day count, larger units would be dekadays, hectodays, and kilodays, replacing a seven-day cycle of activities with a ten-day one. I'm not saying that I think that we should abandon the solar calendar, but a case could be made for ease of calculation of time intervals.

I happen to think that the solar cycle is more significant than the lunar one because of its effects (mostly through weather) on human activity. And I tend to think that the ancients did agree on the significance of the solar cycle, because there are solar calendars and lunisolar calendars, but only one solely lunar one (the Islamic).

When Amsterdam introduced street lamps in 17th century...
those lamps were not lit at full moon. Because unnecessary and wasteful.
A clear sky lit by starlight and skyglow gives about 0,2...0,3 mlux of ground illumination.
Full Moon at zenith gives 270 mlux.
Moon is not at zenith outside tropics, and then not the whole night. But full Moon 12 degrees above horizon and Sun 12 degrees below give by my estimate about 38 mlux of ground illumination (30 mlux from Moon, 8 mlux from twilight), and thatīs the darkest part of full moon. Over 100 times brighter than starlit night.
You are forgetting just how dark a starlit night is and how important moonlight is for daily activities.

While that is true and certainly interesting, Iím not sure it requires adopting a lunisolar calendar rather than simply knowing the phases of the moon.

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Let me clarify my line of thought. I was giving a practical naval example of fagricipni's first case for a possible justification. I cannot think of any additional general arguments. A drawback of the lunisolar calendar as used by the Jews and others is that it is messy in a purely mathematical sense. In ancient times that was a non-issue because their tradition was that a congregation would make the call visually. More recently a council developed a table that works well for centuries at a time. It is by blind luck that a simple leap-year tweak can keep a purely solar calendar in sync with the Sun for several millenia. Thus Caesar found it to be preferable, with a companion ephemeris of the Moon to keep track of the moonlight cycle. Captain Gallery certainly found the Nautical Almanac, along with the Gregorian calendar, entirely satisfactory for facilitating night landings on an aircraft carrier.

When Amsterdam introduced street lamps in 17th century...
those lamps were not lit at full moon. Because unnecessary and wasteful.
A clear sky lit by starlight and skyglow gives about 0,2...0,3 mlux of ground illumination.
Full Moon at zenith gives 270 mlux.
Moon is not at zenith outside tropics, and then not the whole night. But full Moon 12 degrees above horizon and Sun 12 degrees below give by my estimate about 38 mlux of ground illumination (30 mlux from Moon, 8 mlux from twilight), and thatīs the darkest part of full moon. Over 100 times brighter than starlit night.
You are forgetting just how dark a starlit night is and how important moonlight is for daily activities.

It’s true. Standard lighting in car parks is 1 lux and thanks to our rod vision we can easily get around in moonlight at less than that. The Lunar society had good reason to meet under the full moon.

sicut vis videre estoWhen we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.Originally Posted by Ken G

A lunisolar calendar would have the advantage that with a bit of practice you could tell the day of the week just by looking at the moon. The full and new moons and quarters would always be on weekends, or maybe on Moon day. It would require one or two holidays each month so the next month starts with the new moon. It would decouple dates from the seasons by requiring a thirteenth month every three years or so, meaning the solstices and equinoxes would not fall on the same date every year as they do now with our solar calendar.